The Fortress of São João is a historic military fort located in the city of Cascais, Portugal. Built in the late 16th century, it was designed to protect the coastline from potential invasions, particularly from pirates and enemy ships. The fortress features a strategic position overlooking the Tagus River and the Atlantic Ocean. Constructed under the reign of King Philip I of Portugal, the fortress showcases a blend of Mannerist and Renaissance architectural styles. Today, it serves as a cultural landmark, attracting visitors interested in Portugal's maritime history and military architecture.